Valve units having the characteristics indicated above are typically provided for controlling the delivery of fuel gas to a burner or other similar user device for the controlled regulation of its delivery pressure or of the flow-rate of gas supplied.
Valve units of this type are known from the Applicant's own production; such a valve unit typically has a motor-driven actuator for the operative control of a closure member so as to open and close a valve seat formed in the delivery duct. For example, for the control of the closure member, it is known to provide a rod which is connected to the rotor of an electric motor by means of a male-and-female screw coupling in order to move the closure member away from and towards the valve seat as a result of a rotation of the motor. By rotation of the actuator, it is also possible to achieve modulation control of the delivery pressure or of the flow-rate of gas delivered.
To ensure safety closure of the valve seat upon the occurrence of predetermined conditions, for example, in order to shut off the passageway for the gas as a result of the interruption of electrical supply to the motor-driven actuator (in which condition the actuator may stop in an intermediate position of opening of the valve seat), a solution has been provided by the prior art and forms the subject of the Applicant's Italian patent No. PD99A000274. This provides for the use of an electromagnetic unit interposed between the closure member and the main motor-driven actuator and arranged to act on the closure member so as to close the valve seat, by virtue of the resilient force exerted by a spring in opposition to the electromagnetic attraction between the stationary core and the movable armature of an electromagnet of the electromagnetic unit. If conditions arise which require the valve seat to be shut off, the interruption of the electrical supply to the electromagnet brings about the safety closure movement of the closure member, under the action of the above-mentioned resilient force, irrespective of the operative position adopted by the actuator.
Although, on the one hand, this solution is extremely reliable in ensuring the safety closure of the valve seat, on the other hand, it leads to some structural complexity connected mainly with the presence of an electromagnet which floats with the control rod of the closure member and, in particular, which floats inside a region affected by the gas flow. This configuration in fact requires the provision of specific conditions of electrical insulation as well as of sealing with respect to the gas, both of the electrical contacts and of the wires for supplying the electromagnet, which conditions are more difficult to satisfy with an electromagnet which is movable inside the valve unit.